Downholes (underground drilling pits) are prepared for recovery of hydrocarbon resources including oil and gas (representatively called “oil” sometimes hereafter) from the underground, and tools for the formation or repair of the downholes, such as frac plugs (disintegratable plugs), bridge plugs, cement retainers, perforation guns, ball sealers, sealing plugs and packers (inclusively referred to as “downhole tools” hereafter), are used and thereafter disintegrated or allowed to fall down without recovery thereof onto the ground as they are in many cases. (Examples of such downhole tools and manners of use thereof are illustrated in, e.g., Patent documents 1-5). Therefore, about the tool of such temporary use, it has been recommended to form the whole or a component thereof constituting a bonding part allowing collapse (i.e. downhole tool member) with a degradable polymer. Examples of such a degradable polymer may include: polysaccharide, such as starch or dextrin; animal albumin polymers, such as chitin and chitosan; aliphatic polyesters, such as polylactic acid (PLA, typically poly L-lactic acid (PLLA)), polyglycolic acid (PGA), polybutyric acid, and polyvaleric acid; and further, polyamino acids, polyethylene oxide, etc. (Patent documents 1 and 2). However, the technology of designing the mechanical strength and time to collapse of such a downhole tool member by using such a degradable polymer has not satisfactorily developed. This is because it was difficult to accurately evaluate the degradation behavior of the degradable polymer.